Fischer 344 (F344) inbred is the selected rat strain for evaluation of toxic and carcinogenic potential of chemicals. Leukemia is the major cause of mortality for the F344 rat, especially the male. The incidence of leukemia in the F344 rat has increased over the last 15 years and the survival/median life span has decreased. Corn oil or safflower oil, when given by gavage, decreased the incidence of leukemia in the males and increased the survival/median life span. The purpose of this study is to determine the influence of 10% fat diets (NTP-91 diet containing corn oil, and NTP-92 diet containing safflower oil) on the incidence of leukemia and survival when fed to F344 rats for two years. Groups of 60M + 60F F344 rats are fed NIH-07 diet, the standard diet with 5% fat, or the NTP-91 or NTP-92 diets for two years. Body weights, feed consumption and clinical chemistry parameters are determined at selected intervals. Tissues will be collected for evaluation of nonneoplastic and neoplastic changes at the end of the study.